The present invention relates generally to a fuel processor for a hydrogen fuel cell engine, and more specifically to such a processor which uses a combined water gas shift reactor/carbon dioxide (CO2) adsorber.
In proton exchange membrane (PEM) fuel cells, hydrogen (H2) is the anode reactant (i.e. fuel) and oxygen is the cathode reactant (i.e. oxidant). The oxygen can be either a pure form (O2), or air (a mixture of O2 and N2). The solid polymer electrolytes are typically made from ion exchange resins such as perfluoronated sulfonic acid. The anode/cathode typically comprises finely divided catalytic particles, which are often supported on carbon particles, and mixed with a proton conductive resin. The catalytic particles are typically costly precious metal particles. These membrane electrode assemblies are relatively expensive to manufacture and require certain conditions for effective operation. These conditions include proper water management and humidification, and control of catalyst fouling constituents, such as carbon monoxide (CO).
For vehicular applications, it is desirable to use a liquid fuel such as alcohols (e.g., methanol or ethanol), other hydrocarbons (e.g., gasoline), and/or mixtures thereof (e.g., blends of ethanol/methanol and gasoline) as the source of hydrogen for the fuel cell. Such liquid fuels for the vehicle are easy to store onboard, and there is a nationwide infrastructure for supplying liquid fuels. However, such fuels must be dissociated to release the hydrogen content thereof for fueling the fuel cell. The dissociation reaction is accomplished within a chemical fuel processor or reformer. The fuel processor contains one or more reactors wherein the fuel reacts with steam (and sometimes air) to yield a reformate gas comprising primarily hydrogen and carbon dioxide. For example, in the steam methanol reformation process, methanol and water (as steam) are ideally reacted to generate hydrogen and carbon dioxide. In reality, carbon monoxide is also produced requiring additional reaction processes. In a gasoline reformation process, steam, air and gasoline are reacted in a primary reactor which performs two reactions. One is a partial oxidation reaction, where air reacts with the fuel exothermally, and the other is a steam reforming reaction, where steam reacts with the fuel endothermically. The primary reactor produces hydrogen, carbon dioxide, carbon monoxide and water.
Reactors downstream of the primary reactor are required to lower the CO concentration in the hydrogen-rich reformate to levels tolerable in the fuel cell stack. Downstream reactors may include a water/gas shift (WGS) reactor and a preferential oxidizer (PROX) reactor. The PROX selectively oxidizes carbon monoxide in the presence of hydrogen to produce carbon dioxide (CO2), using oxygen from air as an oxidant. Here, control of air feed is important to selectively oxidize CO to CO2. Unfortunately, the preferential oxidation reactor is not 100% selective and results in consumption of hydrogen. The heat generated from the preferential oxidation reactor is at a low temperature, resulting in excess low-grade heat.
The operational gasoline fuel processor technologies to date do not meet automotive targets for start-up durations, mass, and volume. The start-up time for such a system is limited by the time delay until the combination of water gas shift and preferential oxidation reactors can supply stack grade hydrogen. The start-up duration is related to the mass of the catalyst system used for start-up and the energy needed to get the catalyst system up to its operating temperature. Another limitation of the current technology is the inability to utilize the low grade heat such a system generates. Any heat loss reduces the fuel processor efficiency.
There are further drawbacks with current fuel processor systems for hydrogen fuel cell systems. The water gas shift reaction is equilibrium limited, and accordingly, the carbon monoxide concentration leaving high temperature and low temperature water gas shift reactors is typically about 3 mole % and 1 mole %, respectively. Often, a fuel processor will contain a high temperature water gas shift reactor followed by a low temperature water gas shift reactor; or will have two low temperature water gas shift reactors, one running adiabatically and one running isothermally. The carbon monoxide concentration of the reformate must be further reduced to levels that are tolerable in a PEM fuel cell stack, typically less than about 100 ppm and preferably less than about 50 ppm by volume.
The two main methods for removing this carbon monoxide are preferential oxidation (PROX) to carbon dioxide (as discussed hereinabove) and pressure swing adsorption (PSA). Preferential oxidation reactors are difficult to control, and the carbon monoxide cannot be recovered for hydrogen generation. Additionally, air needs to be compressed for the preferential oxidation causing high power requirements, and the nitrogen in the compressed air dilutes the hydrogen product going to the fuel cell stack. In a pressure swing adsorption unit, the carbon monoxide that desorbs leaves the adsorber at a substantially lower pressure than it enters. The carbon monoxide therefore needs to be recompressed in order to be recycled back into the water gas shift reactor(s) for complete carbon monoxide conversion to hydrogen. The PSA system also requires a purge gas and/or a vacuum in order to regenerate the adsorbent.
Both PROX and PSA systems are large, giving an added incentive for eliminating a unit operation from a space-constrained fuel cell vehicle. Furthermore, both water gas shift and PROX reactors need to reach their operational temperatures in order to be efficient for carbon monoxide reduction. Thus, it is difficult to remove carbon monoxide from the reformate upon start up at ambient temperatures in a conventional fuel processing system.
Thus, it is desirable to provide a fuel processor for a hydrogen fuel cell engine which provides a means to eliminate a unit operation from a space-constrained fuel cell vehicle. It is also desirable to provide a means to reduce the carbon monoxide content under normal operation before entering the fuel cell stack, thereby eliminating the use of a preferential oxidizer (PROX) reactor or significantly reducing the size of any such reactor. It is also desirable to provide such a fuel processor which provides quick carbon monoxide uptake during start-up, thereby shortening start-up duration.
The present invention addresses the above-mentioned problems by providing an apparatus for removing carbon monoxide (CO) from a hydrogen-rich gas stream. In one aspect, the hydrogen-rich stream is produced in a hydrogen fuel cell system which further includes membrane electrode assemblies where such hydrogen is reacted with oxygen to produce electricity. CO fouls costly catalytic particles in the membrane electrode assemblies, as described hereinabove. The apparatus comprises a housing which may be a rotating pressure swing adsorber. A catalyst, adapted to perform a water gas shift reaction, is disposed in the housing. A carbon dioxide adsorbent is also disposed in the housing. The adsorption of the carbon dioxide drives the reaction (i.e. carbon monoxide+water ⇄carbon dioxide+hydrogen) toward production of carbon dioxide and hydrogen, thereby advantageously shifting equilibrium toward carbon monoxide consumption.
The apparatus may further comprise a second adsorbent disposed in the housing and adapted to adsorb carbon monoxide from the hydrogen-rich gas stream at low temperatures, and is adapted to desorb carbon monoxide at high temperatures.
The present invention advantageously eliminates a unit operation from a space-constrained fuel cell vehicle by combining the WGS catalyst and a CO2 adsorbent in a single reactor/housing. In addition to eliminating a unit operation, the present invention further eliminates the use of a preferential oxidation (PROX) reactor or significantly reduces the size of any such reactor, by providing an apparatus which incorporates CO2 adsorption (and consequent carbon monoxide consumption) in the place of the PROX reactor. The present invention provides a means to clean up carbon monoxide while minimizing hydrogen consumption and the concomitant, undesirable excess low grade heat generation. The present invention reduces start-up duration, and improves overall fuel processor efficiency during normal operation.